1. Field of the Invention
This invention relates to a polyphonic electronic musical instrument, which can simultaneously generate a plurality of musical tones through a time-division process and, more particularly, to a tone source circuit of such a polyphonic electronic musical instrument.
2. Description of the Related Art
Heretofore, digital electronic musical instruments such as digital synthesizers have been well known in the art, which make use of digital techniques to generate musical tones electronically.
Such digital electronic musical instruments are suited for constructing a polyphonic electronic musical instrument, which can simultaneously generate a plurality of tones, with less circuit elements because the circuit of such instruments can be driven on a time-division basis.
In the prior art polyphonic electronic musical instrument, however, the tone source control data, i.e., sounding on/off signal, frequency data, envelope data and tone waveform access address data, are held in a shift register having a plurality of stages corresponding in number to the number of maximum number of tones that can be generated simultaneously (i.e., number of polyphonic channels). Therefore, the re-writing of data requires a waiting time corresponding to one cycle period of the shift register, which shifts data channel by channel, in the worst case. Although the waiting time does not pose any problem so long as the number of channels is small. However, when the channels are increased in number, the waiting time per channel is increased, leading to a delay of the tone source process after detection of a key-"on" or key-"off".
Further, when effecting a frequency change by providing a pitch bend or vibrato at the time of the tone mixing, the following problem takes place.
It is assumed that a pitch bend operator is operated with respect to the waveform data of each channel at the time of tone mixing of waveform data for sounding by using 1-st and 32-nd channels for a tone. In this case, in response to this operation a plurality of frequency data are successively provided each for each channel. At this time, every time one frequency data is given to each channel, the 1-st and 32-nd channels are turned on for frequency data updating and then turned off. However, since the system has a total of 32 channels, it takes a long time until the process goes from the 1-st channel to the 32-nd channel Therefore, new frequency data is given before the updating of the frequency data of the 32-nd channel after the updating of the frequency data of the 1-st channel. In this case, the preceding frequency data given for the 32-nd channel is skipped to be updated to the new frequency data of this time, that is, the data of the 1-st channel is altered to the preceding frequency data while the data of the 32-nd channel is altered to the frequency data of this time, these altered data being tone-mixed for simultaneous sounding. In this way, the change of data of the tone-mixing channels result in a phase deviation of the frequency data.